Signal sources are a common form of test equipment used in many areas, including the testing of electronic equipment, dynamic structures and audio equipment. One category of test equipment includes sweep generators. Sweep generators are designed to generate a signal (such as a sinusoidal, square, triangular or saw-tooth waves) whose frequency varies in a prescribed manner with respect to time. A common form of sweep is a linear sweep, where the instantaneous frequency increases by a fixed amount per time unit. Another form of sweep is an exponential sweep, in which the frequency increases or decreases exponentially with time. Such as sweep is generated by a “logarithmic sweep generator”, so called because the logarithm of frequency varies linearly with time.
Sweeps are usually described by a set of ‘sweep parameters’, such as the initial and final frequency values and the duration of the sweep.
One form of logarithmic sweep generator uses an analog circuit to produce an exponentially increasing or decreasing voltage, which is then applied to a voltage-controlled oscillator to produce a signal with an exponentially increasing frequency. This approach is limited because of the difficulties in producing circuits that can cover extreme variations of sweep parameters. In addition, the approach is incompatible with digital signal generation techniques, such as fractional-N PLL or Direct Digital Synthesis (DDS).
Another approach stores a list of frequency values in a memory to provide a look-up table. The values are retrieved during a sweep to provide the desired frequency profile. Disadvantages of this method include the size of the memory required for long sweeps with high-resolution steps and the amount of time required for loading the table when the sweep parameters are changed.
A still further approach uses a high-speed processor to compute each new frequency step and updates the frequency at fixed time intervals. This requires large computation power when high resolution (32-bit of higher, for example) frequencies or high step rates are required.